Method and Device for the Operation of a Hydraulic Vehicle

ABSTRACT

A method for the operation of a hydraulic vehicle brake is provided. The brake includes a brake housing in which a hydraulic working pressure chamber is delimited by a displaceable brake piston, which is movable into operative connection with a brake disc in order to achieve a braking effect. A parking brake device acts on the brake piston and, in a condition where the brake piston is in operative connection with the brake disc, is lockable by a locking device. The locking device is activated by a force-transmitting element, which is operable by an electromagnetic actuator with at least one coil, one yoke and one armature, wherein the position of the force-transmitting element connected to the armature is determined because the change in inductance of the coil is determined depending on the armature movement in such a way that the result of the determination is independent of the ambient temperature.

BACKGROUND OF THE INVENTION

The present invention relates to a method for the operation of a hydraulic vehicle brake, including a brake housing in which a hydraulic working pressure chamber is delimited by a displaceable brake piston, which is movable into operative connection with a brake disc in order to achieve a braking effect, and wherein a parking brake device acts on the brake piston and, in a condition where the brake piston is in operative connection with the brake disc, is lockable by means of a locking device, the said locking device being activated by a force-transmitting element, which is operable by means of an electromagnetic actuator comprising at least one coil, one yoke and one armature, and wherein the position of the force-transmitting element connected to the armature is determined because the change in inductance of the coil is determined depending on the armature movement. Further, the invention relates to a device for the operation of a hydraulic vehicle brake.

DE 10 2004 062 810 A1 discloses a hydraulic vehicle brake of this type. In order to determine the position of the force-transmitting element or the armature connected to the force-transmitting element, the change in inductance of the coil in dependence on the armature movement is detected. However, the method known in the art has the shortcoming of inaccuracies.

In view of the above, an object of the invention is to further improve the method as described hereinabove in order to achieve a higher rate of accuracy.

According to the invention, this object is achieved by the method in that the position of the force-transmitting element is determined in such a way that the result of the determination is independent of the ambient temperature.

In a favorable improvement of the method of the invention, the position determination is performed in such a way that the result is irrespective of the temperature of the coil and/or the yoke and the armature.

It is arranged that the position determination is performed with the following steps:

-   -   (i) determining the electric resistance of the coil;     -   (ii) comparing the determined electric resistance with         previously defined values and determining the temperature of the         coil, the yoke, and the armature;     -   (iii) determining the magnetic resistance of the yoke and the         armature;     -   (iv) determining the eddy current in the yoke and in the         armature, and     -   (v) introducing a square-wave voltage signal into the coil and         measuring the current that flows through the coil.

Another, especially favorable embodiment of the method of the invention provides that the temperature-responsive component of the eddy current in the yoke and in the armature is taken into consideration when measuring the current that flows through the coil, and the result of measurement is corrected accordingly. As the eddy current component distorts the current measurement, consideration of the eddy current component and a correction of the result of measurement is especially favorable.

The previously defined values of the electric resistance of the coil are found by calibration. It is arranged in this respect that the calibration takes place before the initiation of the hydraulic vehicle brake and/or in regular intervals during the operation.

In addition, this object is achieved according to the invention in that means are provided, which carry out the position determination of the force-transmitting element in such a way that the result thereof is independent of the ambient temperature, the temperature of the coil, and/or of the yoke and the armature.

In a particularly advantageous improvement of the subject matter of the invention, the means perform the following steps:

-   -   (i) determining the electric resistance of the coil;     -   (ii) comparing the determined electric resistance with         previously defined values and determining the temperature of the         coil, the yoke, and the armature;     -   (iii) determining the magnetic resistance of the yoke and the         armature;     -   (iv) determining the eddy current in the yoke and in the         armature, and     -   (v) introducing a square-wave voltage signal into the coil and         measuring the current that flows through the coil.

The invention will be described in detail hereinbelow by way of an embodiment, making reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an axial cross-sectional view of a hydraulic vehicle brake in which the method of the invention can be implemented;

FIG. 2 is a time diagram showing the current measured in the coil of an electromagnetic actuator;

FIG. 3 is a time diagram showing the eddy current that occurs in the magnetic circuit of an electromagnetic actuator, with different magnetic resistances of the magnetic circuit;

FIG. 4 is a diagram comparing the current measured in the coil with the magnetic resistance in the magnetic circuit at different temperatures;

FIG. 5 is a diagram comparing the magnetic resistance with the air slot between coil and armature of the electromagnetic actuator;

FIG. 6 is a diagram comparing the current measured in the coil with the air slot according to FIG. 5, at different temperatures.

DETAILED DESCRIPTION OF THE DRAWINGS

The hydraulic vehicle brake of the invention shown in FIG. 1 includes a brake housing 1 straddling the outside edge of a brake disc (not shown) and two brake pads (likewise not shown). The brake housing 1 forms on its inside surface a brake cylinder 5 receiving a brake piston 6 in an axially displaceable manner. By way of a hydraulic port 8, brake fluid can be fed into the working pressure chamber 7 formed between brake cylinder 5 and brake piston 6, whereby brake pressure develops that displaces the brake piston 6 axially towards the brake disc. This will urge the brake pad facing the brake piston 6 against the brake disc, whereupon the brake housing 1, as a reaction, displaces in the opposite direction and thereby likewise urges the other brake pad against the brake disc.

As can be taken from FIG. 1 in addition, an energy accumulator 10 is arranged at the side of the brake housing 1 remote from the brake piston 6. Energy accumulator 10 is mainly comprised of a hydraulic accumulator pressure chamber 9, an accumulator piston 11 delimiting the accumulator pressure chamber 9, as well as a spring element 12 being designed as an assembly of cup springs and supported at the accumulator piston 11 in the example shown. The energy stored in the energy accumulator 10 acts on the brake piston 6 during a parking brake operation, as will be explained in more detail in the following. It is hereby achieved that the application force that acts on the brake pads is almost independent of thermally induced changes in length in the area of the brake housing 1.

A spindle drive or a threaded-nut/spindle assembly 14, respectively, forms a locking device, which is necessary for realizing a parking brake function in the design illustrated in FIG. 1. The mentioned threaded-nut/spindle assembly 14 comprises a threaded nut 15 and a spindle 16 being in connection with each other by means of a non-self-locking thread. In this arrangement, the threaded nut 15 is rigidly connected to the brake piston 6, while the spindle 16 at its end remote from the brake piston 6 includes a preferably conical first friction surface 17, which can be moved into and out of engagement with a second friction surface 18 that is arranged in the accumulator piston 11 in a non-rotatable fashion. For this purpose, a force-transmitting element 27 is provided, which is received in a cylindrical stepped bore 13 in the accumulator piston 11, projects through the latter and forms a central bearing 21 for the spindle 16. After a relative movement of the force-transmitting element 27 in relation to the accumulator piston 11, the function of the central bearing 21 is omitted, and the two friction surfaces 17, 18 are in engagement with each other, as will be explained in more detail hereinbelow. Further, a spring 19 supported on the brake housing 1 biases the spindle 16 in the direction of the second friction surface 18 or the central bearing 21, respectively, by the intermediary of an axial bearing 20.

The hydraulic vehicle brake is illustrated in FIG. 1 in the released condition of the parking brake. To lock the parking brake, a pressure generator, not referred to in detail, is used to build up hydraulic pressure initially both in the working pressure chamber 7 and in the accumulator pressure chamber 9. To this end, an electrically operable valve, which is preferably configured as a normally closed (NC) valve 24 must adopt its open operating position. The brake piston 6 displaces to the left in the drawing as a reaction to the pressure buildup in the working pressure chamber 7, while the accumulator piston 11 is displaced to the right in the drawing in opposition to the action of force of the preloaded spring element 12. The spring element 12 is compressed in this action. As this occurs, the accumulator piston 11 entrains the force-transmitting element 27 in that a collar 4 designed at the force-transmitting element 27 is supported at the transition between small and large diameter of the stepped bore 13. The accumulator piston 11 and, hence, the force-transmitting element 27 are displaced to the right due to the above-mentioned pressure buildup in the accumulator pressure chamber 9 until an armature plate 23, which is in a force-transmitting connection with the force-transmitting element 27, moves into abutment with an electromagnetic actuator 3. In this action, the spindle 6 continues bearing against the central bearing 21 due to the action of force of the spring 19, with the result that the two friction surfaces 17, 18 cannot become engaged.

Subsequently, the electromagnetic actuator 3 is energized, with the result that the armature plate 23 is arrested by the electromagnetic actuator 3 in its stop position, as described above. In a following pressure reduction in the working pressure chamber 7 and in the accumulator pressure chamber 9, the brake piston 6 moves to the right in the drawing, while the accumulator piston 11 moves to the left. Arresting of the force-transmitting element 27 enables a relative movement between the force-transmitting element 27 and the accumulator piston 11, whereby the function of the central bearing 21 for the spindle 16 is canceled and the two friction surfaces 17, 18 are moved into engagement with each other. The biased spring element 12 mentioned hereinabove presses the accumulator piston 11, the spindle 16 blocked due to the friction surfaces 17, 18 being in engagement, the threaded nut 15, and thus the brake piston 6 to the left in the drawing and against the brake disc (not shown), respectively. The vehicle brake is thereby locked in its applied condition. Thereafter the electromagnetic actuator 3 is no more energized, and the armature plate 23 and the force-transmitting element 27, respectively, are no more arrested. The valve 24 adopts its de-energized state, and it is hence closed. Thus, the hydraulic vehicle brake does not require energy and hydraulic pressure in order to maintain the locking engagement in the applied condition, which is considered as an advantage.

To release the locking engagement, in turn, hydraulic pressure is built up in the working pressure chamber 7 and, after a corresponding actuation of the NC valve 24, likewise in the accumulator pressure chamber 9. The hydraulic pressure, in turn, would displace the brake piston 6 in FIG. 1 to the left and the accumulator piston 11 to the right. However, it is sufficient for unlocking the parking brake when the accumulator piston 11 is relieved from load. Another spring element 22, which moves the force-transmitting element 27 into abutment at the transition between small and large diameter of the stepped bore 13, urges the force-transmitting element 27 in the direction of the spindle 16 and pushes the engaged friction surfaces 17, 18 open, when the accumulator piston 11 is relieved from load in a corresponding manner. Thereafter, the force-transmitting element 27 forms a central bearing 21 for the spindle 16 again.

As can be seen in FIG. 1, the above-mentioned additional spring element 22 takes care that in the event of a service brake operation, where only the working pressure chamber 7 is acted upon by pressure, the force-transmitting element 27 is not displaced because it is biased by the additional spring element 22 in opposition to the action of force of the hydraulic pressure in the working pressure chamber 7. The accumulator piston 11 is neither displaced in a service brake operation because the effective diameter of the accumulator piston 11 close to the working pressure chamber 7 is smaller than the effective diameter of the brake piston 6. Also, the spring element 12 designed with a preloading force defined by construction acts in opposition to the pressurization in the working pressure chamber 7, what likewise prevents displacement of the accumulator piston 11 during a service brake operation.

The coil 25 of the electromagnetic actuator 3 fulfils the function of a sensor for sensing the position of the armature plate 23, which position allows detecting whether locking of the vehicle brake is or is not possible. In addition, especially the action of the armature plate 23 striking against the electromagnetic actuator 3 is a signal for the pressure generator (not referred to in detail) to terminate the pressure buildup for performing a parking brake operation in the pressure chambers 7, 9. In order to reliably determine the position of the armature plate, the change of inductance of the coil 25 of the electromagnetic actuator 3, being caused by the movements of the armature plate, is defined. This is brought about in that voltage pulses are applied to the coil 25. The variation of the current that flows through the coil 25 is simultaneously determined. This current variation indicates the position of the armature plate 23 and, thus, the position of the force-transmitting element 27. As the position of the armature plate 23 changes, the variation of the current that flows through the coil 25 will change as well. The change of inductance of the coil 25 mainly depends on the size of the slot between the armature plate 23 and the iron yoke 26 of the electromagnetic actuator 3.

As the above-mentioned method for the determination of the position of the armature plate 23 or the force-transmitting element 2 connected to the armature plate 23 exhibits inaccuracies, an object of the invention involves further improving the method described in order to achieve a higher rate of precision. The basic idea is that the magnetic resistance influences the inductance of the coil 25. In the method described hereinbelow, the eddy currents are considered temperature-responsively, whereby higher precision is achieved.

In the following, the principle of measurement of sensing the armature plate distance by measurement of the current rise will be explained in detail. Displacement of the armature plate 23 opens an air slot, which increases the magnetic resistance of the ferromagnetic circuit that is formed of yoke 26 and armature 23. The increase of the magnetic resistance causes a reduction of the inductance. This change shall be measured.

The following applies to each coil in a simplified form:

$\begin{matrix} {{l(t)} = {l_{MAX} \cdot {\left( {1 - ^{- \frac{t}{\tau}}} \right).}}} & (Ι) \\ {{{{{L = {\frac{N \cdot \varphi}{l} = \frac{N^{2}}{R_{Magnetic}}}}{With}\text{}\tau} = \frac{L}{R}},{{l_{MAX} = \frac{U}{R}};{U = {{const}.}};{R = {{const}.}}}}\mspace{14mu}} & ({ΙΙ}) \end{matrix}$

the inductance L can be determined in one single measurement of the current at an appropriate time t, when U and R are known and constant. A graph showing different values of T is illustrated in FIG. 2.

Eddy currents develop in extensive electric conductors and are the result of a change in the magnetic field. The magnetic field of coil 25 is bunched in the magnet bowl and the armature plate 23. The magnetic field is proportional to the current. The magnetic field variation is proportional to the change in current. The eddy current is proportional to the magnetic field variation, and the eddy current magnetic field is proportional to the eddy current.

In addition, as can be seen in the sign, the eddy current magnetic field is inverse to its cause, i.e. the magnetic field of the coil 25. The eddy current in the ferromagnetic circuit 23, 26 is opposed to the current in the coil 25. From this follows that the current rise in the commencement is greater than the mere exponential function.

It applies:

$\begin{matrix} {\varphi = \frac{N \cdot l}{R_{Magnetic}}} & ({III}) \end{matrix}$

From this follows:

$\begin{matrix} {\frac{\varphi}{t} = \frac{N \cdot \frac{l}{t}}{R_{Magnetic}}} & ({IV}) \end{matrix}$

From this follows for the induced voltage in the ferromagnetic circuit:

$\begin{matrix} {U_{ind} = {{- N_{ferro}}\frac{\varphi}{t}}} & (V) \end{matrix}$

The eddy current is a consequence of the induced voltage in the ferromagnetic circuit, with

$\begin{matrix} {{N_{ferro} = {1\mspace{14mu} {and}\mspace{14mu} l_{ind}\frac{U_{ind}}{R_{{el}.{ferro}}}}}\mspace{14mu} {{following}\text{:}}l_{ind} = {\frac{- \frac{\varphi}{t}}{R_{{el}.{ferro}}} = {\frac{\frac{{- N} \cdot \frac{l}{t}}{R_{Magnetic}}}{R_{{el}.{ferro}}} = {\frac{- N}{R_{{el}.{ferro}} \cdot R_{Magnetic}} \cdot \frac{l}{t} \cdot {with}}}}} & ({VI}) \\ {{\frac{l}{t} = {\frac{l_{Max}}{\tau} \cdot ^{- \frac{t}{\tau}}}}{follows}{l_{ind} = {\frac{- N}{R_{{el}.{ferro}} \cdot R_{Magnetic} \cdot \tau} \cdot l_{Max} \cdot ^{- \frac{t}{\tau}}}}} & ({VII}) \end{matrix}$

The induced eddy current in turn produces a magnetic field:

$\begin{matrix} {\varphi_{ind} = {\frac{N_{ferro} \cdot l_{ind}}{R_{Magnetic}} = {\frac{- N}{R_{{el}.{ferro}} \cdot R_{Magnetic}^{2} \cdot \tau} \cdot l_{Max} \cdot ^{- \frac{t}{\tau}}}}} & ({VIII}) \end{matrix}$

The addition of coil field and eddy current field has as a result:

$\begin{matrix} \begin{matrix} {\varphi_{ges} = {{\varphi + \varphi_{ind}} = {\frac{N \cdot l}{R_{Magnetic}} - {\frac{N}{R_{{el}.{ferro}} \cdot R_{Magnetic}^{2}} \cdot \frac{l_{Max}}{\tau} \cdot ^{- \frac{t}{\tau}}}}}} \\ {= {\frac{l_{Max} \cdot N}{R_{Magnetic}} \cdot \left( {1 - ^{- \frac{t}{\tau}} - \frac{^{- \frac{t}{\tau}}}{R_{Magnetic} \cdot R_{{el}.{ferro}} \cdot \tau}} \right)}} \\ {= {\frac{I_{Max} \cdot N}{R_{Magnetic}} \cdot \left( {1 - {^{- \frac{t}{\tau}} \cdot \left( {1 + \frac{1}{R_{Magnetic} \cdot R_{{el}.{ferro}} \cdot \tau}} \right)}} \right)}} \end{matrix} & ({IX}) \end{matrix}$

With

$\begin{matrix} {{l(t)} = {{\varphi (t)} \cdot \frac{R_{magnetic}}{N}}} & (X) \end{matrix}$

follows:

$\begin{matrix} {{l(t)} = {l_{Max} \cdot \left( {1 - {^{- \frac{t}{\tau}} \cdot \left( {1 + \frac{1}{R_{Magnetic} \cdot R_{{el}.{ferro}} \cdot \tau}} \right)}} \right)}} & ({XI}) \end{matrix}$

It can be seen that the function is the original exponential function, with a factor at e. This factor depends on T and thus on R_(Magnetic).

$\begin{matrix} {{\tau = \frac{L}{R_{{el}.{coil}}}}\mspace{14mu} {and}\mspace{14mu} {L = \frac{N^{2}}{R_{Magnetic}}}\mspace{14mu} {hence}\mspace{14mu} {\tau = \frac{N^{2}}{R_{Magnetic} \cdot R_{{el}.{coil}}}}\mspace{14mu} {{follows}\text{:}}\begin{matrix} {{l(t)} = {l_{Max} \cdot \left( {1 - {^{\frac{- \frac{t}{N^{2}}}{R_{Magnetic} \cdot R_{{el}.{coil}}}} \cdot \left( {1 - \frac{R_{{el}.{coil}}}{R_{{el}.{ferro}} \cdot N^{2}}} \right)}} \right)}} \\ {= {\frac{U_{0}}{R_{{el}.{coil}}} \cdot \left( {1 - {^{- \frac{t \cdot R_{Magnetic} \cdot R_{{el}.{coil}}}{N^{2}}} \cdot \left( {1 - \frac{R_{{el}.{coil}}}{R_{{el}.{ferro}} \cdot N^{2}}} \right)}} \right)}} \end{matrix}} & ({XII}) \end{matrix}$

Except for R_(Magnetic) this formula contains only known quantities. FIG. 3 shows a graph for different magnetic resistances. The effect of the eddy current on the time variation I(t) with different magnetic resistances without any temperature influence will be dealt with later on. The eddy current is noticed as a seeming offset of the current. This cannot be monitored in tests. Hence, a component for the model is missing. An ideal inductance can be added to the model in series to the series resistance. It describes the stray and air field, which is not bunched in the ferromagnetic circuit 23, 26 and, consequently, has no eddy current. However, the model is valid for sufficiently great intervals t1, the deviation of the current variation as a function of time then becomes indefinite small.

With temperature influence, the following formula for the current is obtained:

$\begin{matrix} {{l\left( {t;T} \right)} = {{{l_{Max}(T)} \cdot \begin{pmatrix} {1 - {^{\frac{- \frac{t}{N^{2}}}{R_{Magnetic} \cdot R_{{el}.{coil}} \cdot {({1 + {{\alpha_{Cu} \cdot \Delta}\; T}})}}} \cdot}} \\ \left( {1 - \frac{R_{{el}.{coil}} \cdot \left( {1 + {{\alpha_{Cu} \cdot \Delta}\; T}} \right)}{R_{{el}.{ferro}} \cdot \left( {1 + {{\alpha_{Fe} \cdot \Delta}\; T}} \right) \cdot N^{2}}} \right) \end{pmatrix}} = {\frac{U_{0}}{R_{{el}.{coil}} \cdot \left( {1 + {{\alpha_{Cu} \cdot \Delta}\; T}} \right)} \cdot \begin{pmatrix} {1 - {^{- \frac{t \cdot R_{Magnetic} \cdot R_{{el}.{coil}} \cdot {({1 + {{\alpha_{Cu} \cdot \Delta}\; T}})}}{N^{2}}} \cdot}} \\ \left( {1 - \frac{R_{{el}.{coil}} \cdot \left( {1 + {{\alpha_{Cu} \cdot \Delta}\; T}} \right)}{R_{{el}.{ferro}} \cdot \left( {1 + {{\alpha_{Fe} \cdot \Delta}\; T}} \right) \cdot N^{2}}} \right) \end{pmatrix}}}} & ({XIII}) \end{matrix}$

The corresponding formula for the magnetic resistance is:

$\begin{matrix} {{{1 - \frac{{l(t)} \cdot R_{{el}.{coil}}}{U_{0}}} = ^{{- \frac{t \cdot R_{Magnetic} \cdot R_{{el}.{coil}}}{N^{2}}} \cdot {({1 - \frac{R_{{el}.{coil}}}{R_{{el}.{ferro}} \cdot N^{2}}})}}}{\frac{1 - \frac{{l(t)} \cdot R_{{el}.{coil}}}{U_{0}}}{1 - \frac{R_{{el}.{coil}}}{R_{{el}.{ferro}} \cdot N^{2}}} = ^{- \frac{t \cdot R_{Magnetic} \cdot R_{{el}.{coil}}}{N^{2}}}}{{\ln \left\lbrack \frac{1 - \frac{{l(t)} \cdot R_{{el}.{coil}}}{U_{0}}}{1 - \frac{R_{{el}.{ferro}}}{R_{{el}.{ferro}} \cdot N^{2}}} \right\rbrack} = {- \frac{t \cdot R_{Magnetic} \cdot R_{{el}.{coil}}}{N^{2}}}}{R_{Magnetic} = {{- {\ln \left\lbrack \frac{1 - \frac{{l(t)} \cdot R_{{el}.{coil}}}{U_{0}}}{1 - \frac{R_{{el}.{coil}}}{R_{{el}.{ferro}} \cdot N^{2}}} \right\rbrack}} \cdot \frac{N^{2}}{t \cdot R_{{el}.{coil}}}}}} & ({XIV}) \end{matrix}$

R_(Magnetic), R_(el.ferro) and R_(el.coil) are temperature-responsive quantities, however. The temperature of the coil 25 can be measured by way of a calibration at the production site with a known temperature and the I_(Max)-measurement at the beginning of an ignition cycle or an activation, respectively. It must be assumed then that the same temperature is prevailing in the ferromagnetic circuit 23, 26 as well. As the components are positioned in a confined space, this should apply. While relatively simple relations apply with respect to the electric resistances, so far only a rough approximation can be given for the temperature dependence of the magnetic resistance. It is assumed that no temperature dependence prevails in the temperature range to be used. It is applicable for the electric coil resistance:

R _(el.coil) =f(T)=R(25° C.)·(1+αΔT)

For copper, α is at 0.0039 1/K.

The minimum temperature practically is at −40° C., the maximum at +140° C.

Thus, the minimum resistance is at 74.65%, the maximum at 144.48% of the nominal value.

For steel, α is at 0.0048 1/K.

Thus, the minimum resistance is at 68.80%, the maximum at 155.2% of the nominal value.

For R_(Magnetic), it shall be assumed initially that there is no dependence on temperature, at least the dependence at low temperatures is very insignificant.

$\begin{matrix} {R_{Magnetic} = {{- {\ln \left\lbrack \frac{1 - \frac{{l(t)} \cdot R_{{el}.{coil}} \cdot \left( {1 + {{\alpha_{Cu} \cdot \Delta}\; T}} \right)}{U_{0}}}{1 - \frac{R_{{el}.{coil}} \cdot \left( {1 + {{\alpha_{Cu} \cdot \Delta}\; T}} \right)}{R_{{el}.{ferro}} \cdot \left( {1 + {{\alpha_{Fe} \cdot \Delta}\; T}} \right) \cdot N^{2}}} \right\rbrack}} \cdot \frac{N^{2}}{t \cdot R_{{el}.{coil}} \cdot \left( {1 + {{\alpha_{Cu} \cdot \Delta}\; T}} \right)}}} & ({XV}) \end{matrix}$

then applies, and R_(el.coil) is measured directly, because the current for t=100 ms≈t→∞ is measured at a voltage of e.g. 1.8 volt. Thus, the calibration at the production site only takes influence on the assumed temperature of the iron component, i.e. on the calculation of R_(el.ferro)=f(ΔT)=f(l(t=100 ms, U₀=1.8V,T)).

The application of the theoretic formulas is illustrated in FIG. 4 after adaptation of the parameters to measurements. The measured current at time t1=0.002 s is plotted on the x-axis in FIG. 4. Depending on temperature, one of the curves should be selected, and the magnetic resistance can be read on the y-axis.

The same laboratory measurements, by means of which the parameters were found, can be used to produce the function s=f(R_(Magnetic)), a graph of which is shown in FIG. 5.

The polynomial formula can be used to transform each R_(Magnetic), which can now be calculated in a temperature-compensated fashion, into a distance s, which corresponds to the air slot between armature plate 23 and yoke 26 (cf. FIG. 6).

The ohmic resistance of the coil 25 at T0=25° C. can be determined at the production site. Based on diagnosis function, it should also be possible in a workshop to initiate this measurement when replacing the brake caliper. Measuring the ohmic resistance of the coil 25 before the start of a sensing cycle allows determining the temperature of the coil 25 and, hence, of the ferromagnetic circuit 23, 26 as well. The temperature can be used to determine the applicable characteristic curve, thus, there is a defined correlation between current, temperature, on-cycle, voltage, and the result of measurement.

One significant advantage of the method lies in the temperature compensation by measuring the ohmic resistance of the coil 25 at the production site, with the temperature known and prior to each sensing operation. It becomes possible only this way to absolutely measure the armature position. Another advantage involves the mathematical consistency of the solution, a mathematical method of approximation is not necessary. Another advantage lies in the temperature-responsive consideration of the eddy currents, what leads to a higher rate of precision. 

1.-8. (canceled)
 9. A method for the operation of a hydraulic vehicle brake, including a brake housing (1) in which a hydraulic working pressure chamber (7) is delimited by a displaceable brake piston (6), which is movable into operative connection with a brake disc in order to achieve a braking effect, and wherein a parking brake device acts on the brake piston (6) and, in a condition where the brake piston (6) is in operative connection with the brake disc, is lockable by means of a locking device, the said locking device being activated by a force-transmitting element (2), which is operable by means of an electromagnetic actuator (3) comprising at least one coil (25), one yoke (26) and one armature (23), the method comprising the step of measuring a change in inductance of the coil (25) due to an armature movement, calibrating the measured change of inductance with respect to temperature, determining the position of the force-transmitting element (2) connected to the armature (23) based on the calibrated change of inductance.
 10. The method as claimed in claim 9, wherein the calibration is performed in such a way that the result of the position determination is independent of the temperature of at least one member of the group consisting of the coil (25), the yoke (26) and the armature (23).
 11. The method as claimed in claim 9, wherein the position determination is performed with the following steps: (i) determining the electric resistance of the coil (25); (ii) comparing the determined electric resistance with previously defined values and determining the temperature of the coil (25), the yoke (26), and the armature (23); (iii) determining the magnetic resistance of the yoke (26) and the armature (23); (iv) determining the eddy current in the yoke (26) and in the armature (23), and (v) introducing a square-wave voltage signal into the coil (25) and measuring the current that flows through the coil (25).
 12. The method as claimed in claim 11, wherein the temperature-responsive component of the eddy current in the yoke (26) and in the armature (23) is taken into consideration when measuring the current that flows through the coil (25), and the result of measurement is corrected accordingly.
 13. The method as claimed in claim 11, wherein the previously defined values of the electric resistance of the coil (25) are found by calibration.
 14. The method as claimed in claim 13, wherein the calibration takes place before the initiation of the hydraulic vehicle brake and/or in regular intervals during the operation.
 15. A hydraulic vehicle brake system, including a brake housing (1) in which a hydraulic working pressure chamber (7) is delimited by a displaceable brake piston (6), which is movable into operative connection with a brake disc in order to achieve a braking effect, and wherein a parking brake device acts on the brake piston (6) and, in a condition where the brake piston (6) is in operative connection with the brake disc, is lockable by means of a locking device, the said locking device being activated by a force-transmitting element (2), which is operable by means of an electromagnetic actuator (3) comprising at least one coil (25), one yoke (26) and one armature (23), and wherein the position of the force-transmitting element (2) connected to the armature (23) is determined because the change in inductance of the coil (25) is determined depending on the armature movement, wherein a calibration system is provided, which calibrates the position determination of the force-transmitting element (2) in such a way that the result is independent of the temperature ambient temperature, the temperature of at least one member contained in the group consisting of the coil (25), the yoke (26) and the armature (23).
 16. The hydraulic vehicle brake system as claimed in claim 15, wherein the calibration system performs the following steps: (i) determining the electric resistance of the coil (25); (ii) comparing the determined electric resistance with previously defined values and determining the temperature of the coil (25), the yoke (26), and the armature (23); (iii) determining the magnetic resistance of the yoke (26) and the armature (23); (iv) determining the eddy current in the yoke (26) and in the armature (23), and (v) introducing a square-wave voltage signal into the coil (25) and measuring the current that flows through the coil (25). 